"Microsoft recently made a change to the licence agreement saying that a new motherboard is equal to a new computer, hence you need to purchase a new Windows licence. Here is what Microsoft has to say: "An upgrade of the motherboard is considered to result in a 'new personal computer' to which Microsoft OEM operating system software cannot be transferred from another computer. If the motherboard is upgraded or replaced for reasons other than a defect, then a new computer has been created and the license of new operating system software is required." Please note that this does not go for retail copies of Windows, but only for OEM versions.

According to the US Dept of Justice's Findings of Fact document, section III:

"34. Viewed together, three main facts indicate that Microsoft enjoys monopoly power. First, Microsoft's share of the market for Intel-compatible PC operating systems is extremely large and stable. Second, Microsoft's dominant market share is protected by a high barrier to entry. Third, and largely as a result of that barrier, Microsoft's customers lack a commercially viable alternative to Windows."

Apple Mac OS X was originally developed in isolation on a completely different hardware platform. That fact protected it from Microsoft's major attacks (preloads and exclusive deals with hardware makers), and enabled it to survive to the present day. The x86 version is a port of the protected development.

OS/2 was developed on x86 by what was then the largest software company in the world (IBM), but not even IBM could sustain its presence in the market. OS/2 is now unsupported by its maker, and has been in a steady decline markersharewise for over a decade. Keep in mind that IBM also received over US$700 million in compensation for Microsoft's past activities towards OS/2 in the US.

Linux and BSD were mainly developed competely outside of the commercial software marketplace. As noncommercial software development, they are largely immune from the effects of market forces.

Microsoft was in fact found guilty of using anti-competitive behavior to maintain its monopoly on desktop operating systems. You can read U.S. v Microsoft. At no point was the suit dropped; the DoJ under Ashcroft simply didn't seek the same remedies the Reno DoJ had in mind.